1. Field of the Invention
The present invention relates to a digital filter with a combined architecture of a 4-tap real valued filter and a-tap complex valued filter, whereby the filter size is reduced and no output delay is occurred due to the filter operations.
2. Background of the Related Art
A digital filter based on the LMS (Least Mean Square) adaptive algorithm is a filter capable of upgrading or adapting coefficients on an ongoing basis. The LMS adaptive digital filter is usually used for an equalizer or a noise eliminator housed in a digital broadcasting receiver, in order to compensate the distortions generated by a channel or a system itself.
Adaptive filters are largely categorized into two groups: one with real valued signals and real coefficients, and the other with complex valued input signal and complex valued coefficients. Filters with real coefficients and real valued inputs are useful for VSB ground wave digital broadcasts composed of real valued data only. Meanwhile, filters with complex valued input signals and complex valued coefficients are useful for a QAM-based system such as a digital cable broadcasting composed of both real valued and imaginary valued data. To combine the above two systems into one chip, two kinds of filters should be used.
However, when two systems combined in one chip are selectively operated, only one of the filters is used and the other filter does nothing being useless. In the case of a multi-tap filter the filter is usually built in a dual structure, which resultantly enlarges the size of the entire filter, making it more difficult to implement, and system resources are wasted. A filter architecture based on the system computing one tap per symbol clock cannot be also realized if the filter size is large.
Therefore, it will be advantageous to develop a dual system filter combined in one chip where two filters are not operated simultaneously but selectively according to a user's choice, for example, depending on the nature of a system the filter is applied to, the filter functions as a real valued filter with real coefficients or a complex valued filter with complex valued coefficients. Moreover, the entire filter size can be reduced by designing the filter in such a manner that an operator thereof performs computations twice, instead of once as in the related art, per symbol clock.